Ammonia has been widely produced at an industrial level by Haber-Bosch process so far. In Haber-Bosch process, hydrogen and nitrogen are reacted using a doubly promoted iron catalyst under high-temperature and high-pressure condition of 400 to 600° C. and 20 to 40 MPa to obtain ammonia (Non-patent Document 1). In recent years, there is a case that ammonia could be synthesized under a condition of lower temperature and lower pressure by using a Ru-containing catalyst in Haber-Bosch process (Patent Document 1).
As a carrier of a general Ru-containing catalyst used for ammonia synthesis by Haber-Bosch process, a carbon carrier such as an activated carbon (Non-patent Document 3, Patent Document 2 and Patent Document 3), MgO (Non-patent Document 4) and a rare-earth oxide (Non-patent Document 5, Patent Document 1 and Patent Document 4) are used other than Al2O3(Non-patent Document 2). Al2O3 carrier has been widely used as a catalyst carrier and can effectively prevent an active metal from agglomeration. However, an electron donation capability of Al2O3 is weaker than a basic compound, since Al2O3 is a weakly-acidic compound. A basic oxide is exemplified by a carbon material such as an active carbon, MgO and a rare-earth oxide. An electron donation capability of such a basic oxide or a carbon material such as an activated carbon is high. Hence, when the basic oxide or activated carbon is used as a carrier and even a small amount of co-catalyst component is added thereto, a highly active catalyst for synthesizing ammonia can be obtained.
As a co-catalyst component for improving an ammonia generation activity, an alkali metal, an alkaline earth metal and a rare earth are used (Non-patent Document 1). Since a co-catalyst component donates an electron to Ru, an ammonia generation activity is improved.
As a method for synthesizing ammonia, a method described in Patent Document 5 is also proposed. In the method, hydrogen gas and nitrogen gas are supplied, and the reactants are excited by generating low-temperature plasma in the presence of a catalyst.
In such an ammonia synthesis method, it is confirmed that a generation amount of ammonia can be increased by using a catalyst which is prepared by supporting Ru on MgO and adding Cs thereto as a co-catalyst component (Patent Document 6). In addition, a method described in Patent Document 7 is also proposed. In the method, the reactants are ionized using an asymmetric capacitor to synthesize ammonia.
In addition, a technology to inhibit resource depletion and global warming has been demanded. In particular, in a technical field of power generation, use of renewable energy such as solar power and wind power is increased, since renewable energy can suppress an exhaust amount of carbon dioxide, which is one of greenhouse gases, and does not depend on fossil resources. However, it is difficult to obtain stable energy from renewable energy. Hence, a method for stably storing energy for a long time by producing a chemical compound using excess electrical energy or thermal energy derived from renewable energy in order to convert the excess energy into chemical energy is developed (Non-patent Document 6). As a chemical compound which is used for storing energy, a hydrogen-containing compound such as ammonia is proposed (Non-patent Document 7). Ammonia is promising as an energy carrier, since the density of the contained hydrogen per volume and mass of ammonia is high and CO, is not discharged when ammonia is burnt.